The present invention relates to a microwave cooking oven and specifically to an improvement thereof whereby uneven energy distribution within the oven cavity is modified for improved cooking performance.
In a conventional microwave oven cooking cavity the spatial distribution of the microwave energy tends to be non-uniform. As a result, hot spots and cold spots are produced at different locations. For many types of foods, unsatisfactory cooking results because some portions of the food may be completely cooked while others are barely warmed. The problem becomes more severe with foods of low thermal conductivity and low dielectric constant which do not readily absorb microwave energy or conduct heat from the areas which are heated by the microwave energy to those areas which are not. Foods such as cakes fall within this class. However, other foods frequently cooked in microwave ovens, such as meat, also produce unsatisfactory cooking results if the distribution of microwave energy within the oven cavity is not uniform.
One explanation for the non-uniform cooking pattern is that electromagnetic standing wave patterns, known as "modes," are set up within the cooking cavity. When a standing wave pattern is established, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or modal patterns is dependent at least upon the frequency of microwave energy used to excite the cavity and upon the dimensions of the cavity itself. Due to the relatively large number of theoretically possible modes, it is difficult to predict with certainty which of the modes will predominate. The situation is further complicated by the differing loading effects of different types and quantities of food and food containers which may be placed in the cooking cavity.
A number of different approaches to alter the standing wave patterns in the cavity have been tried in an effort to alleviate the problem of non-uniform microwave energy distribution. A common approach involves the use of a so-called "mode stirrer" which typically resembles a fan having metal blades. Normally, the mode stirrer is located in the vicinity of the waveguide oven cavity junction where the microwave energy is coupled from the waveguide into the cavity. The stirrer may be in the cooking cavity itself, in the waveguide near an exit port, or in a recess formed in one of the walls of the cavity, coupling the exit port from the waveguide with the cavity. Mode stirring is an attempt to randomize reflections by introducing time varying scattering of the microwave energy as it enters the cavity. The mode stirring approach provides some improvement to the non-uniform energy distribution problem, but such methods have not proven totally satisfactory. For example, it is still possible to have a region at one side of the cavity at a significantly higher strength than on the opposite side of the cavity. Uneven distribution can also occur in the front to back direction.
U.S. Pat. No. 4,133,997 shows a dual feed system in which energy is admitted to the cavity from waveguide exit ports on opposing side walls. A mode stirrer is located proximate to each exit port. This approach appears to be yet another modification of single feed mode stirrer arrangements, but is still short of being totally satisfactory for cooking foods.
Another approach to achieving more uniform cooking in the oven cavity is to employ a rotating table to support the food. The theory is that as the food is rotated through hot and cold spots in the oven, the time-averaged heating of the food will result in relatively uniform cooking. While somewhat effective, the results depend on the particular mode pattern established in the given oven and on the nature of the food to be cooked. For example, a vertically polarized predominantly TE mode will not perform satisfactorily in cooking horizontally-placed bacon strips despite the use of the rotating table. Also, a mode pattern that produces a low energy level in the center of the oven will cause the axial portion of the rotating food load to remain less well-cooked than the outer regions of the load which pass through the higher energy outer regions in the cavity, as the food rotates.
Yet another approach has involved the use of a rotating antenna in the cavity in an effort to achieve a more uniform heating pattern in the cavity. Prior art relating to such use of rotating antennas may be found in U.S. Pat. Nos. 4,028,521 to Uyeda et al, 4,284,868 to Simpson, and 4,316,069 to Fitzmayer, for example. Even though rotating antennas by themselves read to improve uniformity of energy distribution in the cavity, typical antenna configurations tend to leave cold spots. For centrally mounted antennas, such cold spots tend to occur near the center of rotation of the antenna. Additionally, the portion of the food load facing the antenna tends to cook more than the opposite side of the load, requiring turning of some foods for proper cooking. Thus, while the rotating beam antenna approach provides an improvement over the earlier mode stirrer arrangement, the food cooking performance is still not totally satisfactory.
The use of slotted feed arrangements in microwave ovens is also known in the prior art. Examples include U.S. Pat. Nos. 4,019,009 to Kusonoki et al; 2,704,802 to Blass et al; and 3,810,248 to Risman et al. Slotted feed arrangements of the Kusonoki type use surface wave phenomena for near field heating. Such arrangements tend to primarily heat the portion of the load nearest the slots and thus work well for relatively thin flat loads. For other types of loads, however, the surface waves are supplemented by energy radiated into the cavity from the top or side. Slotted feed arrangements, such as that of Blass et al and Risman et al tend to create standing waves with resultant cold spots at the nodes of the standing wave.
An example of a dual feed system using slots as radiators may be found in U.S. Pat. No. 3,210,511 to Peter H. Smith. The Smith arrangement provides single diametrically opposed slots on the top and bottom walls of the cavity oriented at right angles to each other. Radiation from the slots is 90.degree. out of phase to produce circularly polarized radiation in the cavity. Commonly-assigned, U.S. Pat. No. 4,354,083 of Staats, provides yet another example of a dual feed system using slotted radiators for microwave ovens. The Staats oven employs arrays of slots adjacent the top and bottom cavity walls with a shelf immediately above the bottom slots to heat food supported on the shelf by use of near field heating effects. The top slots radiate microwave energy to illuminate the top portion of the food load.
While the various approaches to the problem of non-uniform energy distribution in microwave oven cavities summarized hereinbefore have achieved varying degrees of success in improving cooking performance, none has proven totally satisfactory in terms of cooking performance and convenience of use.
It is therefore an object of the present invention to provide a microwave oven having an excitation system which provides improved uniformity of time-averaged energy distribution in the oven cavity to more effectively cook even those foods having low thermal conductivity properties, which heretofore have been difficult to cook satisfactorily.
It is a further object of the present invention to provide a microwave oven of the foregoing type which eliminates, or nearly so, the need for manipulating the food load in the cavity during the cooking process.